Heated mattress pad mystery circuit

Thread Starter


Joined Mar 2, 2023
I have two identical queen sized heated mattress pads. One works fine the other has never worked but it's controllers appear to operate normally. I've taken apart the module that's in the defective mattress pad, traced out the circuit and I can't figure out how it ever worked!

It looks like the fault sensing is done by detecting the voltage at the point where the two parallel heating wires connect in series. I'm guessing if the voltage is not evenly divided between the two heating wires a fault is detected by the control modules and power is turned off. However, the series connection of the parallel heating wires is made by two relatively high value resistors, R1(2.4k) and R2(62k) on #2 side and R3(2.4k) and R5(62k) on #1 side. I do not see anyway that the full 120VAC power can be applied to the two in-series 12-ohm (measured cold) heating wires. What am I missing?

BTW I have measured the resistances on the connector pins. The working mattress pad has approximately 150-ohms from BN to BH1 and BN to BH2. Oddly on the defective unit the same measurement gives approximately 6k-ohms on the two sides.